This invention relates to a disc-shaped recording medium used for recording a variety of information signals, such as speech or video signals, and a method for producing the same.
Up to now, a disc-shaped recording medium having a signal recording layer formed on a substrate molded from synthetic resin has been in use. As this type of the disc-shaped recording mediums, there are presently known a magnetic disc for magnetically recording and/or reproducing information signals, a magneto-optical disc for recording and/or reproducing information signals by exploiting changes in the direction of magnetization caused by illuminated light, and an optical disc for recording and/or reproducing the information mainly by optical means.
Such a disc-shaped recording medium, that can be mounted exchangeably on a recording and/or reproducing apparatus, is being used. The exchangeable type disc-shaped recording medium is detachably loaded on a turntable of a disc rotating driving mechanism provided in the recording and/or reproducing apparatus. This disc-shaped recording medium is loaded so as to be unified to the turntable to permit synchronous rotation of the disc-shaped recording medium in unison with the turntable.
There are several systems for uniting the exchange type disc-shaped recording medium on a turntable.
One of such systems is the so-called mechanical securing system in which the disc-shaped recording medium is chucked between the turntable and a chuck member to permit the disc-shaped recording medium to be rotated in unison with the turntable. In this mechanical securing system, a center hole provided in the disc-shaped recording medium is engaged by a centering pin formed on a disc setting surface of the turntable to align the center of rotation of the disc-shaped recording medium with that of the turntable. By coincidence of the center of rotation positions, the disc-shaped recording medium is able to be rotated without producing eccentricities with respect to the turntable. Among the disc-shaped recording mediums loaded on the turntable by the mechanical securing system, there are a compact disc (CD), an optical video disc and a DVD (digital versatile disc).
In the mechanical securing system, in which the turntable and the chuck member need to be arranged on both sides of the disc-shaped recording medium, the recording and/or reproducing apparatus is bulky in size. Moreover, there is required a mechanism for causing the chuck member to be moved into contact with and away from the disc-shaped recording medium on the turntable, thus complicating the mechanism for clamping the disc-shaped recording medium.
Another system is a magnetic clamping system which clamps the disc-shaped recording medium for rotation of the disc-shaped recording medium in unison with the turntable by exploiting the magnetic suction. The disc-shaped recording medium used in this system includes a center opening in which to mount a magnetically attracted metallic clamp member. The metallic clamp member has a center positioning hole in which is engaged the distal end of a spindle shaft carrying the turntable. This disc-shaped recording medium is clamped to the turntable by having the clamp member attracted by a magnetic provided on the turn table and by having the distal end of a spindle shaft engaged in a positioning hole provide in the clamp member. At this time, the center of rotation of the disc-shaped recording medium is caused to coincide with that of the turntable by the distal end of the spindle shaft engaged in the positioning hole provided in the clamp member. Typical of the disc-shaped recording mediums loaded on the disc table by magnetic clamp system is a MO (magneto-optical disc).
In the magnetic clamp system, the clamp member provided on the disc-shaped recording medium is attracted by a magnet provided on the turntable to clamp the disc-shaped recording medium and the turntable so that these will be rotated in unison. Since there is no necessity of providing a chuck member for pressure-supporting the disc-shaped recording medium on the turntable, it is possible to realize a simplified clamp mechanism and a thin type recording and/or reproducing apparatus.
Meanwhile, the disc-shaped recording medium, used in the magnetic clamp system, is clamped by the spindle shaft engaging in a positioning hole formed in the clamp member, with the center of rotation of the disc-shaped recording medium then coinciding with that of the turntable. That is, for correctly recording and/or reproducing information signals, the recording tracks of the disc-shaped recording medium need to be scanned correctly by e.g., an optical head. So, the clamp member needs to be loaded correctly in position at the center of the disc-shaped recording medium. For mounting the clamp member, there is such a method in which the clamp member is secured to the disc-shaped recording medium after the center of the recording track formed in the disc-shaped recording medium is brought on e.g., an X-Y table into coincidence with the center opening formed in the clamp member.
In the case of a disc-shaped recording medium used in the magnetic clamp system in which a clamp member is used both for clamping and for positioning the disc-shaped recording medium on the turntable, it is necessary not only to work the clamp member to high accuracy but also to load the clamp member highly accurately at the center of the disc-shaped recording medium thereby rendering the manufacture extremely difficult. In order to eliminate this deficiency, there is also used a magnet clamp system in which the clamp member is afforded with a function of clamping the disc-shaped recording medium to the turntable and with a function of positioning the disc-shaped recording medium with respect to the turntable. In this magnetic clamp system, a centering member provided centrally for the disc table is engaged in a center opening provided centrally on the disc-shaped recording medium to realize coincidence of the center of rotation of the disc-shaped recording medium and that of the turntable, whilst a metallic clamp member mounted on one surface of the disc-shaped recording medium for closing the center opening is magnetically attracted by a magnet provided on the turntable to clamp the disc-shaped recording medium for rotation in unison with the turntable. Typical of the disc-shaped recording medium loaded on the turntable in accordance with this system is a magneto-optical disc with a diameter of 64 mm.
By designing the disc-shaped recording medium in this manner to realize coincidence of the center of rotation with that of the turntable, it becomes possible to facilitate the working of the clamp member and the mounting thereof on the disc-shaped recording medium. By the centering member on the turntable engaging in the center hole formed in the disc-shaped recording medium, the center of rotation formed in the disc-shaped recording medium can be brought more accurately into coincidence with that of the turntable. That is, since the center opening is formed so that its center is coincident with the center of recording tracks formed on the disc-shaped recording medium, the center of the recording tracks can be directly brought into coincidence with the center of the turntable by the centering member engaging in the center opening.
Meanwhile, in recording and/or reproducing an image file or a computer memory with a larger data volume on or from a disc-shaped recording medium, the data transfer speed needs to be increased, so that the rpm of the disc-shaped recording medium needs to be increased, in contradistinction from the case of recording and/or reproducing the speech or still image as conventionally. For high-speed data processing, the rpm of the disc-shaped recording medium of approximately 4000 to 4500 rpm is required.
If a disc-shaped recording-medium is to be rotated at a constant linear velocity (CLV) to record and/or reproduce information signals, it is rotationally controlled so that the linear velocity of the recording track will be constant at the inner and outer rims of the disc-shaped recording medium. If high-speed seek is performed as the disc-shaped recording medium is rotationally driven at the CLV, the rpm of the spindle motor needs to be changed significantly within a short time depending on the read-out positions of the information signals, that is on the inner or outer track positions of the disc-shaped recording medium.
If the rpm of the disc-shaped recording medium is high or if the rpm of the spindle motor is to be changed significantly within a short time interval, the inertia (inertial force) due to rotation of the disc-shaped recording medium is increased. If the inertia is increased, the disc-shaped recording medium tends to be slipped on e.g. the turntable. That is, since the turntable is generally formed of metal, the substrate of the disc-shaped recording medium is formed of synthetic resin and the turntable has a planar supporting surface for the disc-shaped recording medium, the frictional coefficient between the disc-shaped recording medium and the turntable is reduced, so that slip is liable to be produced.
If the disc-shaped recording medium, run in rotation of the turntable, on which it is loaded, undergoes significant eccentricities, not only is the rotary shaft of the spindle motor loaded excessively, but the disc-shaped recording medium is liable to be slipped to increase errors in the information signals recorded and/or reproduced. If the disc-shaped recording medium undergoes slipping, it ceases to be rotated correctly in synchronism with turntable rotation, with the result that the signal recorded or read out in recording or reproduction undergoes jitter, which increases errors in the information signals recorded and/or reproduced.
If the inertial mass is I and the angular velocity of the rotating disc-shaped recording medium is xcfx89, the moment of inertia Fi of a disc-shaped recording medium run in rotation on a turntable on which it is loaded is represented by the following equation (1):
Fi=I(dxcfx89/dt)xe2x80x83xe2x80x83(1).
If the frictional coefficient between the disc-shaped recording medium and the turntable is xcexc and the force of clamp applied to the disc-shaped recording medium from the perpendicular direction is N, the disc-shaped recording medium begins to slip when the equation (2):
Fi greater than xcexcNxe2x80x83xe2x80x83(2)
is met.
If the substrate of the disc-shaped recording medium is formed of a polycarbonate resin, the inertia produced by the disc-shaped recording medium is as shown in the following Table 1:
Meanwhile, the disc-shaped recording medium, having the substrate thickness of 1.2 m, as shown in the Table, has a uniform thickness of 1.2 mm. A magneto-optical disc, termed an ASxc2x7MO (advantage storage MO), has an outer diameter, that is a diameter, of 122 mm, a substrate thickness in a signal recording portion of 0.6 mm and a thickness of the clamp area of 1.2 mm. The taper disc has a thickness of the clamp area of 1.2 mm and a thickness of the outermost rim of 0.6 mm, and presents a tapered area beginning from the clamp area to the outer rim.
The substrate of the disc-shaped recording medium, shown in Table 1, is formed in its entirety of the polycarbonate resin, manufactured by TEIJIN KASEI KK. under the trade name of AD-5503. It is seen from the results of Table 1 that the lighter the disc-shaped recording medium in weight, the smaller becomes the inertia. In particular, it is seen that the ASxc2x7MO has an inertia of 61.5% of that of the disc-shaped recording medium having a substrate thickness of 1.2 mm. Stated differently, the larger the weight of the disc-shaped recording medium, the larger becomes the eccentricity of the disc-shaped recording medium in rotation, with the result that the slip tends to be produced between it and the turntable.
For preventing the disc-shaped recording medium from slipping when it is loaded on the turntable in position and run in rotation, it is sufficient if the clamp force N to the turntable or the frictional force xcexc with respect to the turntable is increased, as may be seen from the above equation (2).
If the clamp force N of the disc-shaped recording medium to the turntable is increased, the force necessary for detaching the disc-shaped recording medium from the turntable is increased. So, the mechanical strength of the mechanism for loading the disc-shaped recording medium on the recording and/or reproducing apparatus needs to be increased, thus increasing the size of the recording and/or reproducing apparatus. For withstanding the large clamp force, the mounting strength of the clamp member on the disc-shaped recording medium needs to be increased. For increasing the mounting strength of the clamp member, mounting with an adhesive is difficult such that it becomes necessary to mount the clamp member by a mechanical mounting mechanism such as by thermally deforming a portion of the substrate of synthetic resin. With such mounting method, it is difficult to mount the clamp member on the disc-shaped recording medium to high accuracy. Moreover, the mounting process is complicated such that mounting cannot be achieved without considerable difficulties.
For increasing the frictional coefficient xcexc between the disc-shaped recording medium and the turntable, it may be envisaged to provide a material possibly increasing the frictional force, such as a rubber sheet or an O-ring, on the turntable. In such case, parallelism of the turntable surface supporting the disc-shaped recording medium cannot be maintained. If the disc-shaped recording medium is mounted on such turntable, a constant separation cannot be maintained between the disc-shaped recording medium and the recording and/or reproducing head. The reason may possibly be that the rubber sheet or the O-ring by nature cannot be worked or produced with a planar surface to micron-order smoothness, and that, when the disc-shaped recording medium is loaded in position on the turntable, the rubber sheet or the O-ring tends to follow the load in a partialized direction.
The disc-shaped recording medium having a substrate thickness of 1.2 mm but a diameter of 12 cm, or the thin-type disc-shaped recording medium, having a thin substrate thickness, such as ASxc2x7MO, is low in toughness such that it tends to be deformed, e.g., warped, due to temperature differential on storage. In particular, if the disc-shaped recording medium, improved in recording density by increasing the numerical aperture (NA) of the objective lens used for converging a light beam on the disc-shaped recording medium, has become deformed, such as by warping, even to the slightest extent, it cannot scan the desired recording track correctly by the light beam, such that it is no longer able to record and/or reproduce information signals correctly. In this consideration, the disc-shaped recording medium is desirably higher in toughness and exempt from becoming deformed, such as with warping.
Meanwhile, the relation between the thickness and strength against deformation in an optical recording medium is given by the following equation (3):
(strength against deformation)xe2x88x9d(thickness of the disc-shaped recording medium)xe2x80x83xe2x80x83(3).
The disc-shaped recording medium, improved in recording density, needs to be exempt from deformation due to warping to a smaller extent than with the conventional disc-shaped recording medium, as may be seen from the following equation (4):
skew tolerancexe2x88x9dxcex/(NA)2xe2x80x83xe2x80x83(4).
It is extremely difficult to have the warp of the disc-shaped recording medium e.g., within 0.4xc2x0 in view of the skew tolerance. From this it follows that it is crucial to keep the parallelism of the turntable surface carrying the disc-shaped recording medium to maintain the distance between the disc-shaped recording medium and the recording and/or reproducing head.
It is therefore an object of the present invention to provide a novel disc-shaped recording medium capable of resolving the problem inherent in the conventional disc-shaped recording medium, and a method for the preparation thereof.
It is another object of the present invention to provide a disc-shaped recording medium capable of being loaded accurately and run in rotation on a turntable without producing eccentricities.
It is another object of the present invention to provide a disc-shaped recording medium which is strong against environmental changes and less susceptible to deformation, such as warping.
In one aspect, the present invention includes a disc-shaped recording medium including a substrate, a signal recording layer provided on one major surface of the substrate and a clamp member having a positioning unit at a center portion thereof, and formed of a material that can be magnetically attracted by a magnet. The clamp member is unified to the center portion of the substrate so that the surface of the clamp member will be flush with the surface of the substrate.
Preferably, the clamp member has a lug on an outer periphery thereof contacting the substrate. The clamp member is unified to the substrate with the lug nipping into the bulk portion of the substrate.
Preferably, the substrate is comprised of a core and a superficial portion provided at least between the core and the signal recording layer.
Preferably, the disc-shaped recording medium includes a light transmitting layer on a surface of the signal recording layer opposite to the surface thereof facing the substrate.
Preferably, the substrate includes a further superficial portion on a surface thereof opposite to a surface of the core facing the signal recording layer. This further superficial portion is formed of a synthetic resin or a resin composition having the water absorption coefficient not higher than 0.3%.
In another aspect, the present invention provides a method for the preparation of a disc-shaped recording medium including mounting a clamp member at a center portion of a metal mold unit, injecting a resin into the metal mold unit to mold a substrate and taking out the substrate unified to the clamp member when the resin injected into the metal mold unit is at a temperature not higher than the thermal deformation temperature.
On one surface of the substrate, taken out from the metal mold, a signal recording layer is formed to form the disc-shaped recording medium.
In the method for the preparation of the disc-shaped recording medium according to the present invention, the first resin material is injected into the metal mold to mold a superficial portion making up the substrate. After molding this superficial portion, the second resin material is injected into the metal mold to mold the core. The first resin material is then injected into the metal mold for molding a further superficial portion making up the substrate.
In still another aspect, the present invention provides a method for the preparation of a disc-shaped recording medium including mounting a clamp member at a center portion of a metal mold unit, heating the metal mold unit and subsequently placing a sheet member on the metal mold unit, pressuring the sheet member in a direction of pressure bonding the sheet member to the clamp member, and cooling the metal mold unit and subsequently peeling the sheet member unified to the clamp member from the metal mold unit. The metal mold unit is heated to a temperature higher than the glass transition temperature of the sheet member and subsequently the sheet member is placed on the metal mold unit.
Preferably, the metal mold unit is cooled to a temperature not higher than the glass transition temperature of the sheet member and subsequently the sheet member is peeled from the metal mold unit.
In yet another aspect, the present invention provides a disc-shaped recording medium including a substrate, a signal recording layer provided on one major surface of the substrate and a clamp member having a positioning unit at a center portion thereof, and formed of a material that can be magnetically attracted by a magnet. The clamp member is unified to the substrate at the time of molding the substrate.
Other objects, features and advantages of the present invention will become apparent from the following description of preferred embodiments thereof and the claims.